Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1894 edition. Excerpt: ...case the value of the integral is zero, since, owing to the weight of the coil, no movement and, therefore, no speed is imparted to it. We have, therefore, the condition of equilibrium-, = o, where / =--. K is the co-efficient of the instrument obtained by calibration with a continuous current, when the force is given by the deflection of the pointer. Calling this D, and the continuous current c, we have c = KVD and with an alternating current wc have DK = A / P sin 'a dt. The value of the righthand term, as already shown, is-= i4 and we find therefore that = KJfr This is precisely the same formula as with a continuous current, and we see that a dynamometer calibrated on a continuous current may be used for the measurement of an alternating current, and will give the effective value of the current. Now let us see how this instrument may be arranged to measure power. As usually made, the dynamometer has an internal connection between the two coils not accessible to the user. There is thus only one external terminal for the movable coil and one for the fixed coilGenerally there are two fixed coils, in order to obtain a greater range, and then three terminals are provided. For the sake of simplicity we shall however assume that we have to do with an instrument having only one fixed coil, and to avoid complication in the diagram we show the movable and fixed coil to consist of one turn each, though in instruments as actually made, any number of turns may, of course, be used. Let, in fig. 82, C represent the movable and c the fixed coil, whilst df is the internal connection, leaving only the terminals Tx Tt accessible. If the instrument be inserted in the usual way, the current coming in at Tx and going out at Tt will traverse both coils in a...show more